Refactor basemap module on top of xyzservices

[giswqs]

The xyzservices package provides a unified place for XYZ tile management. It would be beneficial to refactor the basemap module based on xyzservies.

References:

• https://github.com/geopandas/xyzservices
• https://github.com/jupyter-widgets/ipyleaflet/issues/854
• https://github.com/geopandas/xyzservices/issues/49

[giswqs]

This feature has been implemented. See notebook example https://leafmap.org/notebooks/27_basemap_gallery

Try it out: https://gishub.org/leafmap-demo

https://user-images.githubusercontent.com/5016453/128418610-320f09c1-4624-4c2c-8b04-1a11dcf32041.mp4

https://user-images.githubusercontent.com/5016453/128418642-4de85f36-910b-44e4-8c82-d9a62ae5c73d.mp4